In a conventional approach, a radio access network (RAN) transmits a page message into a wireless coverage area (such as a cell or sector) where a mobile station may be operating in order to contact the mobile station regarding an incoming call. If the mobile station receives the page message, the mobile station may transmit a response over an access channel. Transmissions over the access channel may be in the form of one or more access probes. Thus, to transmit a response to a page message, a mobile station may transmit a series of access probes in the access channel.
In many cases, the access channel is a shared channel that multiple mobile stations in the wireless coverage area are able to use to transmit messages to the RAN. Thus, when one mobile station is transmitting access probes to respond to a page message, another mobile station may also be transmitting access probes at the same time (either to transmit a response to a page message or to transmit some other type of message). This may cause a “collision” between the mobile stations' access probes, which may result in the RAN being unable to decode the access probes from either mobile station.
Various approaches for reducing the possibility of access probe collisions have been proposed. One such approach is used in Evolution Data-Optimized (EVDO) air interfaces. A recent version of EVDO is described in the following technical specification: 3rd Generation Partnership Project 2, “cdma2000 High Rate Packet Data Air Interface,” 3GPP2 C.S0024-B, v. 2.0 (March 2007), which is fully incorporated herein by reference. In this approach, a mobile station uses a “persistence test” to determine when the mobile station can begin transmitting access probes in the access channel. The persistence test uses a “persistence parameter,” n, that is set by the RAN. To perform the persistence test, the mobile station calculates a persistence probability, p, by the formula, p=2−n/4. See EVDO Specification, page 9-47. The mobile station also generates a pseudorandom number, x, where 0≦x≦1, and compares the values of p and x. See EVDO Specification, pages 9-42 and 13-5. If x<p, the persistence test is said to succeed, and the mobile station may transmit an access probe in the first upcoming access channel cycle. Otherwise, the mobile station waits for a silence period and then repeats the persistence test before transmitting the access probe. See EVDO Specification, page 9-42.
By choosing an appropriate value of n, this approach can reduce the possibility of access probe collisions. For example, n can be chosen so as to set a desirably small probability of having two mobile stations that receive page messages during the same time period both transmitting responses during the same access channel cycle.